The present invention is directed generally to radiocommunication systems and, more particularly, to techniques and structures for transmitting and receiving downlink signals adapted to permit downlink power control and adaptive beamforming in conjunction with half-rate communications.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices.
In North America, digital communication and multiple access techniques such as TDMA are currently provided by a digital cellular radiotelephone system sometimes referred to as the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the interim standard TIA/EIA/IS-54, xe2x80x9cDual-Mode Mobile Station-Base Station Compatibility Standardxe2x80x9d, published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA), which is expressly incorporated herein by reference. Because of a large existing consumer base of equipment operating only in the analog domain with frequency-division multiple access (FDMA), TIA/EIA/IS-54 is a dual-mode (analog and digital) standard, providing for analog compatibility together with digital communication capability. For example, the TIA/EIA/IS-54 standard provides for both FDMA analog voice channels (AVC) and TDMA digital traffic channels (DTC). The AVCs and DTCs are implemented by frequency modulating radio carrier signals, which have frequencies near 800 megahertz (MHz) such that each radio channel has a spectral width of 30 kilohertz (Khz). The IS-54 standard has since been subsumed by the IS-136 standard, which also provides for digital control channels (DCCHs).
In a TDMA cellular radiotelephone system, each radio channel is divided into a series of tinge slots, each of which contains a burst of information from a data source, e.g., a digitally encoded portion of a voice conversation. The time slots are grouped into successive TDMA frames having a predetermined duration. The successive time slots assigned to the same user, which are usually not consecutive time slots on the radio carrier, constitute the user""s digital traffic channel, which may be considered a logical channel assigned to the user. Consider, for example, the exemplary IS-136 DTC frame structure illustrated in FIG. 1. Therein, it can be seen that one 40 ms frame consists of six timeslots.
Since being implemented, IS-136 systems have so far operated only at xe2x80x9cfullxe2x80x9d rate. A full rate DTC according to IS-136 is two timeslots per frame, such that three user""s can be assigned a full rate DTC on each carrier. Thus, for example, terminal A could be assigned a full rate DTC consisting of timeslots 1 and 4 in each frame, terminal B could be assigned a full rate DTC consisting of timeslots 2 and 5 in each frame and terminal C could be assigned a full rate DTC consisting of timeslots 3 and 6 in each frame on a single carrier frequency. However, other systems, e.g., systems operating in compliance with the GSM standard, provide for both full rate and half-rate communication service.
In order to meet the increasing demand for higher system capacity, efforts are now being made to implement half-rate operation in IS-136 systems. A straightforward way to provide half-rate communication service for IS-136 compliant systems is to assign each terminal to one timeslot per frame instead of two, as depicted in FIG. 2. There, each of terminals A-F are assigned one timeslot in each frame. A problem with this solution is that it doubles the delay associated with recovering speech frames in the receiver, e.g., from 40 ms to 80 ms (since each speech frame is interleaved over two timeslots to protect against slow fading).
Another technique which has been proposed for providing half-rate communication service to IS-136 systems is to bit interleave data streams associated with two terminals in each timeslot. Thus, as conceptually illustrated in FIG. 3, data streams associated with terminals A and B are bit interleaved in both timeslots 1 and 4, data streams associated with terminals C and D are bit interleaved in both timeslots 2 and 5, and data streams associated with terminals E and F are bit interleaved in both timeslots 3 and 6. Bit interleaving refers to a form of interleaving wherein every other bit belongs to the same data stream, i.e., ABABABABAB . . . Although this solution reduces the delay associated with recovering speech frames in the receiver, it also has certain drawbacks.
For example, it would be desirable, in future IS-136 compliant systems, to provide downlink power control. Currently, all timeslots on an IS-136 downlink carrier are transmitted at a constant power level by the base station. However, it is generally desirable to tailor the base station""s transmit power for each connection to be only that which is necessary to provide a desired quality of service (QoS) as measured by, for example, a signal-to-noise ratio (SNR) experienced by a mobile station. For TDMA systems, downlink power control implies varying the power associated with transmissions to different mobile stations which are receiving signals in each frame. For example, as shown in FIG. 4, it may be desirable to transmit bits to mobile station A(which is relatively close to the base station 40) at a lower power level than those bits which are transmitted to mobile station B (which is more distant from the base station). However, if bit interleaving is used to enable half-rate communications, downlink power control would be precluded since the base station cannot ramp up/ramp down transmit power on a bit-by-bit basis.
Another problem associated with bit interleaving is that it negates the adavantages which can be realized using adaptive beamforming. Array antennas are being provided to base stations in modern radiocommunication systems which antennas provide for spatial xe2x80x9csteeringxe2x80x9d of signal energy in the direction of each recipient mobile. For example, as illustrated in FIG. 5, a base station 50 employing an array antenna can direct the downlink signal energy in one or more of a plurality of beams (only some of which are shown in FIG. 5). Thus, to minimize interference, it would be desirable to only transmit in beam 52 when sending mobile station A""s bits and only transmit in beam 54 when sending mobile station B""s bits. If, however, data streams associated with two mobile stations are bit interleaved within a timeslot for half-rate communications, it will not be possible to selectively transmit mobile station A""s bits only in one or more beams directed toward mobile station A and selectively transmit mobile station B""s bits only in one or more beams directed toward mobile station B.
Accordingly, it would be desirable to provide a communication technique, and systems associated therewith, which would enable half-rate communication in a manner which was also conducive to enabling downlink power control and/or selective transmission in the appropriate beam(s) using an adaptive array antenna.
These and other drawbacks and limitations of conventional techniques and systems are overcome by exemplary embodiments of the present invention wherein data streams transmitted to multiple terminals are multiplexed in a manner which promotes downlink power control and adaptive beamforming. For example, in support of half-rate communications in an IS-136 compliant system, payload data associated with a first terminal can be provided in one or more data fields in a first half of a timeslot and payload data associated with a second terminal can be provided in one or more data fields in a second half of a timeslot. A synchronization/training field can be placed at or near the middle of the timeslot. This concept can be extended to multiplex more than two terminals in a timeslot, e.g., by grouping bits associated with a terminal""s data stream in a portion of each timeslot.
Using timeslot formats according to the present invention provides the capability to multiplex multiple terminals in each timeslot and also provide downlink power control. For example, while a base station is transmitting a first portion of a timeslot to a first mobile station the base station can transmit with a first power level which is tailored to support the connection between the base station and the first mobile station. Then, when the base station is transmitting a second portion of the same timeslot to a second mobile station, the base station can transmit with a second power level (e.g., different than the first power level) which is tailored to support the connection between the base station and the second mobile station. Between the first and second portions of the timeslot, there may be a common synchronization/training/pilot field which can be used by both the first and second mobile stations to process their payload data in that timeslot and which, preferably, is therefore transmitted at the larger of the first and second power levels.
According to other exemplary embodiments of the present invention, multiple terminals can have payload information multiplexed in a single timeslot and selective transmission of signal energy in one or more beams associated with each terminal is also possible. Using an array antenna, a base station can determine a direction (or location) of a terminal which is transmitting on the uplink. Then, for example, while a base station transceiver is transmitting a first portion of a timeslot to a first mobile station the base station can transmit only in one or more beams associated with the first mobile station. Then, when the base station transceiver is transmitting a second portion of the same timeslot to a second mobile station, the base station can transmit signal energy only in one or more beams associated with the second mobile station. Between the first and second portions of the timeslot, there may be a common synchronization/training/pilot field which can be used by both the first and second mobile stations to process their payload data in that timeslot and which, preferably, is therefore transmitted in beams associated with both the first and second mobile stations.
These embodiments may, of course be combined such that the base station transceiver adjusts both its power level and beam steering depending upon, for example, the location of a mobile station to which it is transmitting in each timeslot.